Transistors typically include semiconductor regions that are used to form the source regions and drain regions. The contact resistance between metal contact plugs and the semiconductor regions is high. Accordingly, metal silicides are formed on the surfaces of the semiconductor regions such as silicon regions, germanium regions, and silicon germanium regions in order to reduce the contact resistance. The contact plugs are formed to contact the silicide regions, and the contact resistance between the contact plugs and the silicide regions is low.
A typical silicidation process includes forming a metal layer on the surfaces of the semiconductor regions, and then performing an annealing, so that the metal layer reacts with the semiconductor regions to form the silicide regions. After the reaction, the upper portions of the metal layer may be left un-reacted. An etching step is then performed to remove the un-reacted portions of the metal layer. Contact plugs are then formed to contact the silicide regions.
With the increasing down-sizing of integrated circuits, the silicide regions, and hence the contact between the contact plugs and the silicide regions, also become increasingly smaller. Accordingly, the contact resistance of the electrical contacts becomes increasingly higher. For example, in Fin Field-Effect Transistors (FinFETs), the fins are very narrow, causing the contact areas between the contact plugs and the fins to be very small. The contact resistance to the source and drain regions of the FinFETs thus becomes an increasingly severe problem.